1. Field
The present disclosure relates to a method for preparing monolithic silica aerogel. More particularly, the present disclosure relates to a method for preparing monolithic silica aerogel, including hydrophobitizing monolithic wet silica gel by dipping it into an alkylsilane solution.
2. Description of the Related Art
Silica aerogel, which is given many attentions recently as an ultralight advanced material, is a material having unlimited applicability to industrial fields of energy and environment by virtue of its physical properties, such as high porosity, large surface area, low density, transparency and low heat conductivity. Therefore, porous silica aerogel is applicable to industrial fields related to heat insulating materials, noise-protecting materials, storage materials, ultralight materials for cars and space crafts, electrochemical materials, catalysts of electronic materials, catalyst carriers, or the like. Thus, it is expected that silica aerogel is useful as a key material in various industrial fields.
Particularly, use of silica aerogel as a heat insulating material is one of the most commercially practical uses of silica aerogel. Transparent silica aerogel may be used as a heat insulating window, while opaque silica aerogel may be used effectively as a heat insulating material for various low-temperature or high-temperature heat insulating materials. Transparent aerogel allows transmission of solar light and effective shielding of heat, and thus may provide an energy-saving window system when used for a skylight. Currently, skylight ceiling window systems having a double pane window in which translucent silica aerogel particles are filled have been commercialized and distributed for practical use. However, such systems have a limitation in transparency and are problematic in that the aerogel particles are driven downwardly due to the gravity during long-time use. Meanwhile, monolithic transparent aerogel may be filled into double pane windows to be used as heat insulating windows. However, except some specialized uses, it is difficult to commercialize such windows in terms of cost efficiency, because silica aerogels are required to be formed into monoliths having the same size as the windows.
Monolithic hydrophilic silica aerogel shows transparency and high heat insulating property, but is sensitive to moisture in the air. Thus, such silica aerogel causes cracking on the surface and inner part of the aerogel when exposed to the air for a long time, thereby making it difficult to maintain its originally high heat insulating property. Therefore, it is required to provide a method for preventing moisture absorption in the atmosphere for the purpose of commercialization of such aerogel. For this, many studies have been conducted and many methods have been suggested to provide hydrophobic aerogel.
Particularly, the following methods for preparing hydrophobic silica aerogel have been suggested. First, Korean Patent Laid-Open publication No. 2011-0125773 discloses a method for preparing hydrophobic silica aerogel, which includes preparing a sol solution by using a tetraethoxysilane precursor and an alcohol solvent, and introducing hexamethyldisilazane during the synthesis of gel. This is a general method for imparting hydrophobic property to silica aerogel. However, the method results in a rapid increase in shrinkage of silica aerogel and degradation of heat conductivity.
Next, U.S. Pat. No. 5,888,425 discloses a method for preparing hydrophobic silica aerogel, which includes preparing silicatic lyogel, subjecting the lyogel to a solvent exchange with another organic solvent, reacting the gel with a chlorine-free silylating agent to hydrophobitize it via alkyl radical reaction, and subjecting the resultant gel to subcritical drying. WO 98/02336 discloses a method for preparing hydrophobic silica aerogel, which includes reacting water glass with acid to form lyogel, subjecting the lyogel to a solvent exchange with another organic solvent, silylating the gel by using disiloxane, and subjecting the resultant gel to drying. As such, the above methods essentially require a solvent exchange with an organic solvent and use of an excessive amount of silylating agent for hydrophobitization, and thus have poor cost efficiency.
In addition, the process of hydrophobitization in the methods for preparing hydrophobic silica aerogel according to the related art has a difficulty in application to monolithic aerogel, is complicated due to the use of mixed solution after pH adjustment, and shows poor cost efficiency due to the continuous use of expensive butanol and alkylsilylating agent from a reflux process to a hydrophobitization operation. Further, the drying operation at high temperature after the hydrophobitization causes problems of high shrinkage and degradation of heat conductivity.
To enhance the applicability of hydrophobic monolithic silica aerogel, it is important to provide aerogel to which hydrophobic property is imparted while minimizing deformation of aerogel. According to the related art, silica aerogel is hydrophobitized by using an excessive amount of silylating agent. Thus, the related art is not cost efficient and is problematic in that it causes an increase in shrinkage and degradation of heat insulating property during the hydrophobitization. Under these circumstances, there is a need for providing hydrophobic monolithic silica aerogel having excellent heat insulating property by imparting hydrophobic property thereto in a more cost-efficient manner.